J Comp Physiol B (1992) 162:1-4 Journal of Comparative sy~,m~,'~176 and Environ- Physiology B "'' Physiology Springer-Verlag 1992 Polyunsaturated dietary lipids lower the selected body temperature of a lizard Fritz Geiser 1'*, Bruce T. Firth 2, and Roger S. Seymour 3 1 Department of Zoology, University of New England, Armidale, New South Wales 2351, Australia 2 Department of Anatomy and Histology, and 3 DePartment of Zoology, University of Adelaide, Adelaide, South Australia 5001, Australia Accepted September 27, 1991 Summary. Cold acclimation lowers the selected body temperature (Tb) in many ectothermic vertebrates. This change in behavioural thermoregulation is accompanied by an increase in the proportion of polyunsaturated fatty acids in tissues and cellular membranes. We investigated how diets containing different fatty acids, known to sig- nificantly alter the fatty acid composition of animal tis- sues and membranes, affect the selected Tb of the lizard Tiliqua rugosa. Lizards on a diet containing many pol- yunsaturated fatty acids (10% sunflower oil) showed a 3-5 ~ decrease in Tb, whereas Tb in animals on a diet containing mainly saturated fatty acids (10% sheep fat) did not change. Our study suggests that the composition of dietary lipids influences thermoregulation in ectother- mic vertebrates and may thus play a role in the seasonal adjustment of their physiology. Key words: Fatty acids - Diet - Behavioural thermoregu- lation - Reptiles - Lizard - Tiliqua rugosa Introduction Terrestrial vertebrates have to cope with diurnal and annual fluctuations of environmental temperatures. Ho- meothermic mammals and birds avoid fluctuations of body temperature (TD) by maintaining thermal homeo- stasis via internal heat production. A thermally constant internal milieu promotes metabolic efficiency and in- dependence from external temperature change. In con- trast, cellular function in ectothermic vertebrates is sub- ject to variations in the thermal environment, but many species are able to impart some control of Tb by behav- ioural thermoregulation (Huey 1982). To adjust for seasonal changes in environmental tem- perature many ectothermic vertebrates lower the selected Abbreviations: CST, central standard time; T~, air temperature; T~, body temperature * To whom offprint requests should be sent Tb during winter (Hazel and Prosser 1974; Case 1976; Rismiller and Heldmaier 1988). This seasonal alteration in the selected Tb requires biochemical alterations of cellular components to maintain normal function at the lower Tb. Such modifications seem to be achieved pre- dominantly by changes in the lipid composition of tissues and cell membranes (Cossins et al. 1977; Hazel 1988). Cold acclimation increases the concentration of polyun- saturated fatty acids in tissue fats and phospholipids of cellular membranes of ectotherms which seems impor- tant for maintenance of suitable fluidity and permeability of membranes at low Tb (Cossins and Bowler 1987; White and Somero 1982). However, the polyunsaturated fatty acids linoleic acid (C18:2) and linolenic acid (C18:3) are essential in the diet of vertebrates and are required for production of most longer chain polyun- saturated fatty acids (Lehninger 1982). Therefore an in- crease in the proportion of polyunsaturated fatty acids in body lipids can only be achieved by either selecting a diet of appropriate composition or by the selective incor- poration of these fatty acids. Because cold acclimation increases the proportion of polyunsaturated fatty acids in tissues and cellular mem- branes of ectothermic vertebrates, and because dietary lipids also alter tissue and membrane lipid composition of animals (McMurchie 1988; Geiser 1990), we were interested in whether a diet rich in polyunsaturated fatty acids ("unsaturated" diet; Table 1) would lower the selected T b of shingle-back lizards Tiliqua rugosa, in comparison to animals on a diet rich in saturated fatty acids ("saturated" diet; Table 1). Materials and methods Twelve T. rugosa (Scincidae) were caught in November near Mur- ray Bridge, South Australia. They were divided into two groups of matched body mass and sex ratio and kept at a constant air tem- perature (T,) of 20 ~ photoperiod 05:00-19:00 hours CST. One day after capture the selected T~ was measured (pre-feeding) for 24 h in a 1.5-m thermal gradient with substrate (sand) temperature gradually changing from 7 to 57 ~ photoperiod 05:00-19:00 h.